1. Field of the Invention
Embodiments of the present invention relate generally to optical communication systems and components and, more particularly, to a delay-line interferometer with a liquid crystal tuning element.
2. Description of the Related Art
Differential phase-shift keying (DPSK) is a form of phase modulation used in optical communication systems that conveys data in an optical signal by changing the phase of the carrier wave. Optical communication systems commonly use signal phase modulation, and particularly DPSK, for transmission since phase modulation is strongly resistant to signal degradation caused by chromatic dispersion or polarization mode dispersion in a transmission line. In order to convert such a phase-keyed signal into an amplitude-keyed signal, i.e., an optical-intensity-keyed signal, delay line interferometers are used in optical communication systems as signal demodulators. Delay line interferometers are commonly Mach-Zehnder or Michelson interferometers based on multiple beam interference, in which one beam is time-delayed with respect to another by a desired time interval to produce an interference intensity. Typically, an incoming DPSK optical signal is split into two substantially equal-intensity beams in two arms of the delay line interferometer, where one beam is delayed by a given optical path difference corresponding to a one-bit time delay in the DPSK scheme. The beams are recombined in the delay line interferometer to produce constructive or destructive interference in the time domain for each bit. The resultant interference intensity is the amplitude-keyed signal.
As the bit rates used in optical communication systems increase into the gigabits per second (Gbps) regime, optical systems capable of accurately demodulating phase modulated signals become increasingly difficult to manufacture due to the precise alignment and sizing of optical components and coatings required. Accordingly, there is a need in the art for delay line interferometers that provide the requisite optical performance for Gbps optical communication systems using combinations of basic and easy-to-manufacture optical components.